Substrate holder, substrate support and method of clamping a substrate to a clamping system

ABSTRACT

A substrate holder for use in a lithographic apparatus and configured to support a substrate, the substrate holder including a main body having a main body surface, a plurality of main burls projecting from the main body surface, wherein each main burl has a distal end surface configured to support the substrate, a first seal member projecting from the main body surface and having an upper surface, the first seal member surrounding the plurality of main burls and configured to restrict the passage of liquid between the substrate and the main body surface radially inward past the first seal member; and a plurality of minor burls projecting from the upper surface of the first seal member, wherein each minor burl has a distal end surface configured to support the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT PatentApplication No. PCT/EP2018/079185, which was filed Oct. 24, 2018, whichclaims the benefit of priority of European patent application no.17202627.0 which was filed on Nov. 20, 2017 and which is incorporatedherein in its entirety by reference.

FIELD

The present invention relates to a method of clamping a substrate to aclamping system, to a substrate holder for use in a lithographicapparatus and configured to support a substrate on a substrate support,as well as to a substrate support.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) of a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

As semiconductor manufacturing processes continue to advance, thedimensions of circuit elements have continually been reduced while theamount of functional elements, such as transistors, per device has beensteadily increasing over decades, following a trend commonly referred toas ‘Moore's law’. To keep up with Moore's law the semiconductor industryis chasing technologies that enable to create increasingly smallerfeatures. To project a pattern on a substrate a lithographic apparatusmay use electromagnetic radiation. The wavelength of this radiationdetermines the minimum size of features which are patterned on thesubstrate. Typical wavelengths currently in use are 365 nm (i-line), 248nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extremeultraviolet (EUV) radiation, having a wavelength within a range of 4 nmto 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smallerfeatures on a substrate than a lithographic apparatus which uses, forexample, radiation with a wavelength of 193 nm.

In a lithographic apparatus a substrate is held on a substrate holder.It is desirable that the substrate is as flat as possible to minimiseimaging errors that could be incorporated due to any deviation fromperfect flatness. One difficulty is that when the substrate is initiallyplaced onto the substrate holder the substrate may be non-flat. Once onthe substrate holder, the substrate has in-plane stresses in it andburls of the substrate holder which support the substrate areelastically deformed. These in-plane shear stresses in the substrateresult in in-plane deformations in the substrate which themselves resultin overlay errors.

SUMMARY

It is an object of the present invention to provide a method, substrateholder and substrate support incorporating a substrate holder in whichmeasures are taken to encourage a substrate placed on the substrateholder to relax.

In an embodiment of the present invention there is provided a method ofclamping a substrate to a clamping system, the method comprising thesteps of: providing a substrate holder comprising; a main body having afirst main body surface and a second main body surface, wherein thefirst main body surface and second main body surface are on oppositesides of the main body; and a plurality of first burls projecting fromthe first main body surface, wherein each first burl has a distal endsurface configured to support the substrate; providing a support surfacefor supporting the substrate holder; providing a plurality of secondburls for supporting the substrate holder on the support surface throughcontact with distal end surfaces of the plurality of second burls:generating a first force to attract the substrate holder to the supportsurface; placing the substrate on the substrate holder such that itcontacts the plurality of first burls; generating a second force toattract the substrate to the substrate holder; and controlling at leastone of the first force and the second force in a release step to deformthe main body between the second burls such as to create a gap betweenthe distal end surfaces of a first subset of the plurality of firstburls and the substrate and such that the substrate is supported ondistal end surfaces of a second subset of the plurality of first burls.

In an embodiment of the present invention, there is provided a substrateholder for use in a lithographic apparatus and configured to support asubstrate on a substrate support, the substrate holder comprising: amain body having a first main body surface and a second main bodysurface, wherein the first main body surface and the second main bodysurface are on opposite sides of the main body; a plurality of firstburls projecting from the first main body surface, wherein each firstburl has a distal end surface configured to support the substrate; and aplurality of second burls projecting from the second main body surface,wherein each second burl has a distal end surface for supporting thesubstrate holder on the substrate support wherein the distal endsurfaces of a first subset of the plurality of first burls are a firstdistance from the first main body surface and the distal end surfaces ofa second subset of the plurality of first burls are a second distancefrom the first main body surface, the first distance is more than thesecond distance.

In an embodiment of the present invention, there is provided a substratesupport comprising: a substrate holder comprising: a main body having afirst main body surface and a second main body surface, wherein thefirst main body surface and second main body surface are on oppositesides of the main body; and a plurality of first burls projecting fromthe first main body surface, wherein each first burl has a distal endsurface configured to support the substrate; and a support surface forsupporting the substrate holder through contact with distal end surfacesof a plurality of second burls projecting from the support surface;wherein the distal end surfaces of a first subset of the plurality offirst burls are a first distance from the first main body surface andthe distal end surfaces of a second subset of the plurality of firstburls are a second distance from the first main body surface, the firstdistance is more than the second distance.

In an embodiment of the present invention, there is provided a substrateholder for use in a lithographic apparatus and configured to support asubstrate on a substrate support, the substrate holder comprising: amain body having a first main body surface and a second main bodysurface, wherein the first main body surface and the second main bodysurface are on opposite sides of the main body; a plurality of firstburls projecting from the first main body surface, wherein each firstburl has a distal end surface configured to support the substrate; aplurality of electrodes to which voltages may be applied in order tosecure the substrate holder to the substrate support; wherein theplurality of electrodes are configured such that depending on thevoltage applied, a force between portions of the main body of thesubstrate holder underneath a first subset of the plurality of firstburls and the substrate support is controllable independently of a forcebetween portions of the main body of the substrate holder underneath asecond subset of the plurality of first burls and the substrate support.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIGS. 2a and 2b depict, in cross section, two different versions of afluid handling structure with different features illustrated on the lefthand side and the right hand side, which may extend around the completecircumference;

FIG. 3 illustrates, in cross-section, a substrate holder, substrate andsubstrate support of an embodiment;

FIG. 4 illustrates an alternative embodiment of substrate holder andsubstrate support, in a relaxed state;

FIGS. 5-7 illustrate the steps of placing a substrate onto the substrateholder supported by the substrate support of FIG. 3 or 4;

FIG. 8 illustrates, in cross-section, an alternative embodiment of thesubstrate holder, substrate and substrate support of FIG. 3;

FIGS. 9 and 10 illustrate, in cross-section, an alternative embodimentof the substrate holder, substrate and substrate support of FIG. 3.

FIG. 11 illustrates schematically a pattern of first and second burlsnot falling within the scope of the present invention.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus. The lithographicapparatus includes an illumination system (also referred to asilluminator) IL configured to condition a radiation beam B (e.g., UVradiation or DUV radiation), a mask support (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device MA in accordance with certain parameters, a substratesupport (e.g., a substrate table) WT constructed to hold a substrate(e.g., a resist coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate support WT in accordancewith certain parameters, and a projection system (e.g., a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives the radiation beam Bfrom a radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus may be of a type wherein at least a portionof the substrate may be covered by an immersion liquid having arelatively high refractive index, e.g., water, so as to fill a space 11between the projection system PS and the substrate W—which is alsoreferred to as immersion lithography. More information on immersiontechniques is given in U.S. Pat. No. 6,952,253, which is incorporatedherein by reference.

The lithographic apparatus may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus maycomprise a measurement stage. The measurement stage is arranged to holda sensor and/or a cleaning device. The sensor may be arranged to measurea property of the projection system PS or a property of the radiationbeam B. The measurement stage may hold multiple sensors. The cleaningdevice may be arranged to clean part of the lithographic apparatus, forexample a part of the projection system PS or a part of a system thatprovides the immersion liquid. The measurement stage may move beneaththe projection system PS when the substrate support WT is away from theprojection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

To clarify the invention, a Cartesian coordinate system is used. TheCartesian coordinate system has three axis, i.e., an x-axis, a y-axisand a z-axis. Each of the three axis is orthogonal to the other twoaxis. A rotation around the x-axis is referred to as an Rx-rotation. Arotation around the y-axis is referred to as an Ry-rotation. A rotationaround about the z-axis is referred to as an Rz-rotation. The x-axis andthe y-axis define a horizontal plane, whereas the z-axis is in avertical direction. The Cartesian coordinate system is not limiting theinvention and is used for clarification only. Instead, anothercoordinate system, such as a cylindrical coordinate system, may be usedto clarify the invention. The orientation of the Cartesian coordinatesystem may be different, for example, such that the z-axis has acomponent along the horizontal plane.

Immersion techniques have been introduced into lithographic systems toenable improved resolution of smaller features. In an immersionlithographic apparatus, a liquid layer of immersion liquid having arelatively high refractive index is interposed in a space 11 between aprojection system PS of the apparatus (through which the patterned beamis projected towards the substrate W) and the substrate W. The immersionliquid covers at least the part of the substrate W under a final elementof the projection system PS. Thus, at least the portion of the substrateW undergoing exposure is immersed in the immersion liquid. The effect ofthe immersion liquid is to enable imaging of smaller features since theexposure radiation will have a shorter wavelength in the liquid thangas. (The effect of the immersion liquid may also be regarded asincreasing the effective numerical aperture (NA) of the system and alsoincreasing the depth of focus.)

In commercial immersion lithography, the immersion liquid is water.Typically the water is distilled water of high purity, such asUltra-Pure Water (UPW) which is commonly used in semiconductorfabrication plants. In an immersion system, the UPW is often purifiedand it may undergo additional treatment steps before supply to theimmersion space 11 as immersion liquid. Other liquids with a highrefractive index can be used besides water can be used as the immersionliquid, for example: a hydrocarbon, such as a fluorohydrocarbon; and/oran aqueous solution. Further, other fluids besides liquid have beenenvisaged for use in immersion lithography.

In this specification, reference will be made in the description tolocalized immersion in which the immersion liquid is confined, in use,to the space 11 between the final element 100 and a surface facing thefinal element 100. The facing surface is a surface of substrate W or asurface of the supporting stage (or substrate support WT) that isco-planar with the surface of the substrate W. (Please note thatreference in the following text to surface of the substrate W alsorefers in addition or in the alternative to the surface of the substratesupport WT, unless expressly stated otherwise; and vice versa). A fluidhandling structure 12 present between the projection system PS and thesubstrate support WT is used to confine the immersion liquid to theimmersion space 11. The space 11 filled by the immersion liquid issmaller in plan than the top surface of the substrate W and the space 11remains substantially stationary relative to the projection system PSwhile the substrate W and substrate support WT move underneath.

Other immersion systems have been envisaged such as an unconfinedimmersion system (a so-called ‘All Wet’ immersion system) and a bathimmersion system. In an unconfined immersion system, the immersionliquid covers more than the surface under the final element 100. Theliquid outside the immersion space 11 is present as a thin liquid film.The liquid may cover the whole surface of the substrate W or even thesubstrate W and the substrate support WT co-planar with the substrate W.In a bath type system, the substrate W is fully immersed in a bath ofimmersion liquid.

The fluid handling structure 12 is a structure which supplies theimmersion liquid to the immersion space 11, removes the immersion liquidfrom the space 11 and thereby confines the immersion liquid to theimmersion space 11. It includes features which are a part of a fluidsupply system. The arrangement disclosed in PCT patent applicationpublication no. WO 99/49504 is an early fluid handling structurecomprising pipes which either supply or recover the immersion liquidfrom the space 11 and which operate depending on the relative motion ofthe stage beneath the projection system PS. In more recent designs, thefluid handling structure extends along at least a part of a boundary ofthe space 11 between the final element 100 of the projection system PSand the substrate support WT or substrate W, so as to in part define thespace 11.

The fluid handling structure 12 may have a selection of differentfunctions. Each function may be derived from a corresponding featurethat enables the fluid handling structure 12 to achieve that function.The fluid handling structure 12 may be referred to by a number ofdifferent terms, each referring to a function, such as barrier member,seal member, fluid supply system fluid removal system, liquidconfinement structure, etc.

As a barrier member, the fluid handling structure 12 is a barrier to theflow of the immersion liquid from the space 11. As a liquid confinementstructure, the structure confines the immersion liquid to the space 11.As a seal member, sealing features of the fluid handling structure forma seal to confine the immersion liquid to the space 11. The sealingfeatures may include an additional gas flow from an opening in thesurface of the seal member, such as a gas knife.

In an embodiment the fluid handling structure 12 may supply immersionfluid and therefore be a fluid supply system.

In an embodiment the fluid handling structure 12 may at least partlyconfine immersion fluid and thereby be a fluid confinement system.

In an embodiment the fluid handling structure 12 may provide a barrierto immersion fluid and thereby be a barrier member, such as a fluidconfinement structure.

In an embodiment the fluid handling structure 12 may create or use aflow of gas, for example to help in controlling the flow and/or theposition of the immersion fluid.

The flow of gas may form a seal to confine the immersion fluid so thefluid handling structure 12 may be referred to as a seal member; such aseal member may be a fluid confinement structure.

In an embodiment, immersion liquid is used as the immersion fluid. Inthat case the fluid handling structure 12 may be a liquid handlingsystem. In reference to the aforementioned description, reference inthis paragraph to a feature defined with respect to fluid may beunderstood to include a feature defined with respect to liquid.

A lithographic apparatus has a projection system PS. During exposure ofa substrate W, the projection system PS projects a beam of patternedradiation onto the substrate W. To reach the substrate W, the path ofthe radiation beam B passes from the projection system PS through theimmersion liquid confined by the fluid handling structure 12 between theprojection system PS and the substrate W. The projection system PS has alens element, the last in the path of the beam, which is in contact withthe immersion liquid. This lens element which is in contact with theimmersion liquid may be referred to as ‘the last lens element’ or “thefinal element”. The final element 100 is at least partly surrounded bythe fluid handling structure 12. The fluid handling structure 12 mayconfine the immersion liquid under the final element 100 and above thefacing surface.

FIGS. 2a and 2b show different features which may be present invariations of fluid handling structure 12. The designs may share some ofthe same features as FIGS. 2a and 2b unless described differently. Thefeatures described herein may be selected individually or in combinationas shown or as required.

FIG. 2a shows a fluid handling structure 12 around the bottom surface ofa final element 100. The final element 100 has an invertedfrusto-conical shape. The frusto-conical shape having a planar bottomsurface and a conical surface. The frusto-conical shape protrudes from aplanar surface and having a bottom planar surface. The bottom planarsurface is the optically active portion of the bottom surface of thefinal element 100, through which the radiation beam B may pass. Thefinal element 100 may have a coating 30. The fluid handling structure 12surrounds at least part of the frusto-conical shape. The fluid handlingstructure 12 has an inner-surface which faces towards the conicalsurface of the frusto-conical shape. The inner-surface and the conicalsurface have complementary shape. A top surface of the fluid handlingstructure 12 is substantially planar. The fluid handling structure 12may fit around the frusto-conical shape of the final element 100. Abottom surface of the fluid handling structure 12 is substantiallyplanar and in use the bottom surface may be parallel with the facingsurface of the substrate support WT and/or substrate W. The distancebetween the bottom surface and the facing surface may be in the range of30 to 500 micrometers, desirably in the range of 80 to 200 micrometers.

The fluid handling structure 12 extends closer to the facing surface ofthe substrate W and substrate support WT than the final element 100. Aspace 11 is therefore defined between the inner surface of the fluidhandling structure 12, the planar surface of the frusto-conical portionand the facing surface. During use, the space 11 is filled withimmersion liquid. The immersion liquid fills at least part of a bufferspace between the complementary surfaces between the final element 100and the fluid handling structure 12, in an embodiment at least part ofthe space between the complementary inner-surface and the conicalsurface.

The immersion liquid is supplied to the space 11 through an openingformed in a surface of the fluid handling structure 12. The immersionliquid may be supplied through a supply opening 20 in the inner-surfaceof the fluid handling structure 12. Alternatively or additionally, theimmersion liquid is supplied from an under supply opening 23 formed inthe undersurface of the fluid handling structure 12. The under supplyopening 23 may surround the path of the radiation beam B and it may beformed of a series of openings in an array. The immersion liquid issupplied to fill the space 11 so that flow through the space 11 underthe projection system PS is laminar. The supply of the immersion liquidfrom the opening 23 under the fluid handling structure 12 additionallyprevents the ingress of bubbles into the space 11. This supply of theimmersion liquid functions as a liquid seal.

The immersion liquid may be recovered from a recovery opening 21 formedin the inner-surface. The recovery of the immersion liquid through therecovery opening 21 may be by application of an under pressure; therecovery through the recovery opening 21 as a consequence of thevelocity of the immersion liquid flow through the space 11; or therecovery may be as a consequence of both. The recovery opening 21 may belocated on the opposite side of the supply opening 20, when viewed inplan. Additionally or alternatively, the immersion liquid may berecovered through an overflow opening 24 located on the top surface ofthe fluid handling structure 12. In an embodiment, the supply andrecovery openings 20, 21 can have their function swapped (i.e. the flowdirection of liquid is reversed). This allows the direction of flow tobe changed depending upon the relative motion of the fluid handlingstructure 12 and substrate W.

Additionally or alternatively, immersion liquid may be recovered fromunder the fluid handling structure 12 through a recovery opening 25formed in its bottom surface. The recovery opening 25 may serve to hold(or ‘pin’) a meniscus 33 of the immersion liquid to the fluid handlingstructure 12. The meniscus 33 forms between the fluid handling structure12 and the facing surface and it serves as border between the liquidspace and the gaseous external environment. The recovery opening 25 maybe a porous plate which may recover the immersion liquid in a singlephase flow. The recovery opening in the bottom surface may be a seriesof pining openings 32 through which the immersion liquid is recovered.The pining openings 32 may recover the immersion liquid in a two phaseflow.

Optionally radially outward, with respect to the inner-surface of thefluid handling structure 12, is an gas knife opening 26. Gas may besupplied through the gas knife opening 26 at elevated speed to assistliquid confinement of the immersion liquid in the space 11. The suppliedgas may be humidified and it may contain substantially carbon dioxide.Radially outward of the gas knife opening 26 is a gas recovery opening28 for recovering the gas supplied through the gas knife opening 26.Further openings, for example open to atmosphere or to a gas source, maybe present in the bottom surface of the fluid handling structure 12. Forexample, further openings may be present between gas knife opening 26and gas recovery opening 28 and/or between pining openings 32 and gasknife opening 26.

Features shown in FIG. 2b which are common to FIG. 2a share the samereference numbers. The fluid handling structure 12 has an inner surfacewhich complements the conical surface of the frusto-conical shape. Theundersurface of the fluid handling structure 12 is closer to the facingsurface than the bottom planar surface of the frusto-conical shape.

Immersion liquid is supplied to the space 11 through supply openings 34formed in the inner surface of the fluid handling structure 12. Thesupply openings 34 are located towards the bottom of the inner surface,perhaps below the bottom surface of the frusto-conical shape. The supplyopenings 34 are located around the inner surface, spaced apart aroundthe path of the radiation beam B.

Immersion liquid is recovered from the space 11 through recoveryopenings 25 in the undersurface of the fluid handling structure 12. Asthe facing surface moves under the fluid handling structure 12, themeniscus 33 may migrate over the surface of the recovery opening 25 inthe same direction as the movement of the facing surface. The recoveryopenings 25 may be formed of a porous member. The immersion liquid maybe recovered in single phase. In an embodiment the immersion liquid isrecovered in a two phase flow. The two phase flow is received in achamber 35 within the fluid handling structure 12 where it is separatedinto liquid and gas. The liquid and gas are recovered through separatechannels 36, 38 from the chamber 35.

An inner periphery 39 of the undersurface of fluid handling structure 12extends into the space 11 away from the inner surface to form a plate40. The inner periphery 39 forms a small aperture which may be sized tomatch the shape and size of the radiation beam B. The plate 40 may serveto isolate the immersion liquid at either side of it. The suppliedimmersion liquid flows inwards towards the aperture, through the inneraperture and then under the plate 40 radially outwardly towards thesurrounding the recovery openings 25.

In an embodiment the fluid handling structure 12 may be in two parts asshown on the right hand side of FIG. 2b : an inner part 12 a and anouter part 12 b. The inner part 12 a and the outer part 12 b may moverelatively to each other, in a plane parallel to facing surface. Theinner part 12 a may have the supply openings 34 and it may have theoverflow recovery 24. The outer part 12 b may have the plate 40 and therecovery opening 25. The inner part 12 a may have an intermediaterecovery 42 for recovering the immersion liquid which flows between theinner part 12 a and the outer part 12 b.

The present invention will be described below with reference to animmersion lithographic apparatus. In such an apparatus imaging of thesubstrate W is performed with most parts of the apparatus surrounded bya gaseous atmosphere. A substrate support WT, as described below, insuch an apparatus can make the use of one or more underpressures inorder to apply a force to the substrate W to hold it in place. However,the present invention is applicable to other types of lithographicapparatus. For instance, the invention is suitable for use in anyapparatus where imagining of the substrate W is carried out in a gaseousatmosphere. Additionally the invention can be applied to an EUVlithographic apparatus in which imaging of the substrate W is carriedout in a vacuum. In that instance, instead of using underpressures togenerate a holding force on the substrate W, electrostatic forces areused to hold the substrate W in place. This will be described withreference to FIGS. 9,10 and 11.

The substrate support WT comprises a substrate holder 200 which isconfigured to support the substrate W. FIG. 3 illustrates, incross-section a substrate holder 200 and the associated substrate W anda substrate support WT according to an embodiment. FIG. 3 illustratesthe situation in which the substrate W is clamped ready for imaging. Thesubstrate holder 200 comprises a main body 210 having a first main bodysurface 212. In use the first main body surface 212 faces anundersurface of the substrate W.

In a central region of the first main body surface 212, a plurality offirst burls 220 project from the first main body surface 212. Each firstburl 220 has a distal end surface configured to support the substrate W.The first burls 220 are arranged relative to another in a first regularpattern, in plan. The first regular pattern is such as to support thesubstrate W and to reduce any bowing of the substrate W towards or awayfrom the main body surface 212 to an acceptable amount.

The area in plan of each first burl 220 is relatively small compared tothe area, in plan, of the substrate W. Therefore the first burls 220contact only a small area of the undersurface of the substrate W. Thisreduces the opportunity for contamination to be transferred from thesubstrate holder 200 to the substrate W.

The main body 210 has a second main body surface 214. The second mainbody surface 214 is on an opposite side of the main body 210 to thefirst main body surface 212. In use the second main body surface 214faces a support surface 300 of the substrate support WT.

In a central region of the second main body surface 214 a plurality ofsecond burls 240 project from the second main body surface 214. Eachsecond burl 240 has a distal end surface configured to support thesubstrate holder 200 on the support surface 300 of the substrate supportWT through contact with the support surface 300. The second burls 240are arranged relative to another in a second regular pattern, in plan.The second regular pattern is such as to support the substrate holder200.

The substrate holder 200 and substrate support WT together make up aclamping system 1000. The clamping system 1000 is adapted to be able toextract gas from between the first main body surface 212 and thesubstrate Was well as from between the second main body surface 214 andthe support surface 300 under the control of a clamping systemcontroller 110. The clamping system controller 110 is configured tocontrol the pressure of gas between the first main body surface 212 andsubstrate W independently of the pressure of gas between the second mainbody surface 214 and the support surface 300.

Similarly a pressure differential across the substrate holder 200 may beestablished. For example, the space between the main body 210 of thesubstrate holder 200 and the support surface 300 of the substratesupport WT is evacuated to an underpressure that is lower than a higherpressure above the substrate W. The pressure difference gives rise to afirst force holding the substrate holder 200 to the substrate supportWT.

A pressure differential across the substrate W is established. Forexample, the space between the main body 210 of the substrate holder 200and the substrate W is evacuated to an under pressure that is lower thana higher pressure above the substrate W. The pressure difference givesrise to a second force holding the substrate W to the substrate holder200.

The precise arrangement of first burls 220 and second burls 240 will bedescribed below. However it will be apparent from FIG. 3 that in anembodiment the number of second burls 240 is less than the number offirst burls 220. The first burls 220 are split into two or more subsets.For simplicity the present invention is described in relation to havinga first subset 220 a of the plurality of first burls 220 and a secondsubset 220 b of the plurality of first burls 220. However the inventioncan be extended to having further subsets of the plurality of firstburls 220.

The substrate holder 200 is arranged such that the first subset 220 a ofthe plurality of first burls 220 are moveable in the z direction towardsand away from the substrate W relative to the second subset 220 b of theplurality of first burls 220. Two general ways of achieving this will bedescribed. The first way is that illustrated in FIGS. 3 and 4 which usesunderpressures to generate a force between the substrate holder 200 andthe substrate support WT and between the substrate holder 200 and thesubstrate W. The second way is illustrated with reference to FIGS. 8, 9,and 10 where electrodes are used to generate electrostatic forcesbetween the substrate holder 200 and substrate support WT and betweenthe substrate holder 200 and the substrate W. Hybrid systems which useone or both underpressure and electrostatic forces are also possible.

In the embodiment of FIG. 3, the second subset 220 b of the plurality offirst burls 220 each has an axis 230 which is a distance 237 from anaxis 235 of one of the plurality of second burls 240 which is smallerthan the distance 239 between an axis of a burl of the first subset 220a of the plurality of first burls 220 and the closest axis 235 of one ofthe plurality of second burls 240. In other words, each burl of thesecond subset 220 b of the plurality of first burls 220 has acorresponding second burl 240. In an embodiment, such as for exampleillustrated in FIG. 3 if the distance 237 were zero, a central axis 235of each second burl 240 is coaxial with a central axis 230 of acorresponding one of the first burls 220. The alignment of the secondsubset 220 b of the plurality of first burls 220 with second burls 240means that force from the substrate W on the substrate holder 200 istransferred down to the substrate support WT through both the burls ofthe second subset 220 b and the second burls 240. In other words thestiffness of the substrate holder 200 at a position corresponding topositions of each of the burls of the second subset 220 b of theplurality of first burls 220 is greater than a stiffness of thesubstrate holder 200 at a position corresponding to positions of each ofthe burls of the first subset 220 a of the plurality of first burls 220.The axes 235 and 230 could be co-planar. Other ways of achieving thisdifference in stiffness are possible, for example by varying thethickness of the main body 210 and/or varying the dimensions of secondburls 240 (smaller second burls 240 providing lower stiffness).

In an embodiment, the number of burls of the second subset 220 b issubstantially equal to the number of the plurality of second burls 240.

The embodiment of FIG. 4 is the same as the embodiment of FIG. 3 exceptas described below. In the embodiment of FIG. 4, the second burls 240are part of the substrate support WT and project from the supportsurface 300. The substrate holder 200 and substrate support WT of theFIG. 4 embodiment works in the same way as that of the embodiment ofFIG. 3. The embodiment of FIG. 3 could also have the second burls 240projecting from the support surface 300 rather than from the second mainbody surface 214.

As illustrated in FIG. 4, the substrate holder 200 is in a relaxed statein which no forces are being applied to it. The substrate holder 200 ofthe embodiment of FIG. 3 is similar, in the relaxed state. In thisconfiguration the distance which the distal ends of the burls of thefirst subset 220 a project from the first main body surface 212 is lessthan the corresponding distance which the burls of the second subset 220b project from the first main body surface 212. The difference indistance is illustrated by gap 222 illustrated in FIG. 4. The reason forthis is the above mentioned property that the stiffness of the substrateholder 200 at a position corresponding to the burls of the first subset220 a is lower than at positions corresponding to burls of the secondsubset 220 b. Therefore if the same force is applied attracting thesubstrate W to the substrate holder 200 over the entire area, the mainbody 210 will bow at positions corresponding to the first subset ofburls 220 a towards the substrate W. If the burls of the first subset220 a and the second subset 220 b were the same distance from the firstmain body surface 212, this would result in the burls of the firstsubset 220 a standing proud of the burls of the second subset 220 b. Inorder to make sure that when the substrate W is clamped in positionready for imaging that the distal end surfaces of all of the first burls220 are in the same plane, the distance which the distal end surfaces ofthe burls of the first subset 220 a project from the first main bodysurface 212 is less by an amount illustrated by gap 222 than thedistance which the distal end surfaces of the burls of the second subset220 b project from the first main body surface 212. As a result, in theclamped state, the substrate W can be held flat and in contact with allof the first burls 220 as illustrated in FIG. 3. In the situationillustrated in FIG. 7 and as described below, the bowing of the mainbody 210 at a position corresponding to a burl of the first subset 220 ais illustrated schematically whereas in FIG. 3 this bowing is notillustrated.

If a lower pressure is applied between the main body 210 and thesubstrate support WT than between the main body 210 and the substrate W,deformation of the main body 210 towards the substrate support WT mayoccur between the second burls 240. As a result burls of the firstsubset 220 a will move slightly downwards towards the substrate supportWT and away from the substrate W. This allows the substrate W to besupported in this condition by fewer first burls 220, namely by burls ofthe second subset 220 b, than would be the case either if the pressureabove and below, as illustrated in FIG. 3, the main body 210 were equalor if each of the first burls 220 had a corresponding second burl 240.This principle is used in the invention relating to a method of clampinga substrate, as discussed below with reference to FIGS. 5-7. First thesubstrate W is loaded onto the substrate support 200 in a way such asillustrated in FIG. 3. The substrate W may initially be bowed (up(umbrella) or down (bowl)). In this configuration because of the highfriction between the distal end surfaces of the first burls 220 and thesubstrate W, elastic deformation of the first burls 220 in the xy planeis present. This elastic deformation results in in-plane deformation ofthe substrate W. By continuing to hold the substrate W but releasing onesubset of the two subsets 220 a, 220 b of the plurality of first burls220, any elastic deformation of the burls of the released subset 220 a,220 b can relax. The burls of the released subset of burls can then bere-engaged with the substrate W. In the particular configurationillustrated in FIGS. 3-7 it is then possible to release the other subset220 a, 220 b of burls from contact with the substrate W such that theycan also relax. This process may be repeated as many times as isdesirable, each iteration resulting in lower in-plane stress in thesubstrate W. Finally the substrate W may be fully clamped ready forimaging by engaging the distal end surfaces of all of the first burls220 with the substrate W, as illustrated in FIG. 7.

FIGS. 3 and 5 to 7 show how the clamping system 1000 can be used duringloading of the substrate W on the substrate holder 200 and to enable thesubstrate W to relax more fully than would be the case if the main body210 does not bow between the second burls 240. The same principles canbe applied to the clamping system of FIG. 4.

An underpressure is generated between the main body 210 and the supportsurface 300 to generate a first force to attract the substrate holder200 to the support surface 300. A second force is generated to attractthe substrate W to the substrate holder 200 by generating anunderpressure between the substrate W and the substrate holder 200. Thesubstrate W is then loaded onto the substrate holder 200 as illustratedin FIG. 3.

The magnitudes of the first and second force are controlled to result ina release step in which the main body 210 moves towards the supportsurface 300 in areas under burls of the first subset 220 a (the centralfirst burl 220 a as illustrated in FIG. 5) to create a gap 225 betweendistal end surfaces of burls of the first subset 220 a and the substrateW. The absence of second burls 240 below burls of the first subset 220 ameans that the relatively thin main body 210 will deform between thematching sets of burls of the second subset 220 b and second burls 240when force with which the substrate holder 200 is attracted to thesubstrate support WT is increased. The deformation of the main body 210is downwards towards the substrate support WT away from the substrate W.As a result the distal end surface of the burls of the first subset 220a positioned above the less stiff portion of the main body 210 whichdeforms towards the substrate support WT moves out of contact with theundersurface of the substrate W. This means that it is possible tosupport the substrate W on the substrate holder 200 in a condition inwhich not all of the first burls 220 are in contact with theundersurface of the substrate W.

As a result of the burls of the first subset 220 a coming out of contactwith the substrate W, in plane stresses in the burls of the first subset220 a are relaxed. Similarly, any in-plane stresses in the substrate Wresulting from contact between the burls of the first subset 220 a andthe substrate W are relaxed in the substrate W. Thereby in-planedeformations of the substrate W are reduced resulting in better overlayperformance.

After the substrate W has released, as illustrated in FIG. 5, the burlsof the first subset 220 a which do not have a corresponding second burl240 are still distal from the undersurface of the substrate W. That is,a gap 225 exists between the distal end surface of the burls of thefirst subset 220 a and the undersurface of the substrate W.

At this stage, the controller 110 begins a re-engagement step byincreasing the underpressure (i.e. reduces absolute pressure) betweenthe first main body surface 212 and the substrate W. This is effectiveto increase the second force to attract the substrate W to the substrateholder 200 as the pressure under the substrate W is lower than thepressure above the substrate W. Alternatively or additionally theunderpressure between the second main body surface 214 and the supportsurface 300 may be decreased (i.e. increases the absolute pressure)thereby to decrease the first force. Both the first and second force maybe varied. Thus the situation moves to a situation in which all of thefirst burls 220 contact the undersurface of the substrate W.

In a disengagement step the underpressure between the first main bodysurface 212 and the substrate W is increased and/or the underpressurebetween the second main body surface 214 and the support surface 300 isdecreased. That is in the disengagement step the first force is reducedand/or the second force is increased. This results in a gap 226developing between the distal end surfaces of the burls of the secondsubset 220 b and the substrate W. This is due to the main body 210bowing away from the support surface 300 at locations corresponding tothe burls of the first subset 220 a as a result of the change in thefirst and/or second force. That is, the substrate W is then only supportby burls of the first subset 220 a, as illustrated in FIG. 6. In thisposition any in-plane elastic deformations of the burls of the secondsubset 220 b can relax and any associated in-plane stresses in thesubstrate W at the positions of the burls of the second subset 220 b andalso relax.

The final stage, namely a further re-engagement step, is illustrated inFIG. 7. Here a step of controlling the first and/or second force suchthat the of the burls of the first subsets 220 a and the second subset220 b contact the substrate W and hold it flat ready for imagining isperformed. For instance, the pressure between the main body 210 and thesupport surface 300 is the same as the pressure between the main body210 and the substrate W. During the further re-engagement step, thefirst force is increased and/or the second force is reduced. As aresult, because of the lower stiffness of the main body 210 at aposition of burls of the first subset 220 a, the main body 210 deformstowards the substrate W between the second burls 240. Due to thedifference in height, i.e., the gap 222 in the relaxed state of thesubstrate holder 200 between burls of the first subset 220 a and thesecond subset 220 b, the bowing results in the distal end surfaces ofburls of both the first subset 220 a and the second subset 220 b endingup in substantially the same plane such that the substrate W can be heldflat whilst being in contact with all of the first burls 220. In thisway in-plane stresses in the substrate W are relaxed. Therefore in theposition illustrated in FIG. 7 all of the undersurface of the substrateW is supported at the position of each of the first burls 220 and thesubstrate W can be imaged.

The steps of releasing illustrated in FIG. 5, re-engagement whichhappens between FIGS. 5 and 6, disengagement as illustrated in FIG. 6and further re-engagement as illustrated in FIG. 7 can be performed asmany times as is desired, each iteration resulting in a furtherreduction in in-plane stresses in the substrate W.

Although described above with reference to increasing and decreasingunderpressures, the first and/or second force can be generated in otherways, for example electrostatically for example using Coulomb clampingor Johnson-Rahbek clamping for example by applying a voltage to one ormore electrodes. In a further embodiment, the first and/or second forcecan be generated or at least controlled by a combination of applying anunderpressure and electrostatically.

In order for the clamping system 1000 to be able independently togenerate pressures in the space between the second main body surface 214and the support surface 300 and between the first main body surface 212and the substrate W it is desirable that the passage of gas from a firstside of the substrate holder 200 to the second side of the substrateholder 200 through the main body 210 is prevented. Therefore if anythrough holes are provided through the main body 210 it must be possiblefor these through holes to be blocked. For example, pins may be providedin the substrate support WT which are actuatable (not depicted in thefigures). The pins can be actuated such that they protrude away from thesubstrate support WT more than the first burls 220. During loading andunloading of the substrate W, the pins are extended and the substrate Wis supported by the pins. The pins are then retracted thereby to placethe substrate W onto the distal end surfaces of the first burls 220which have a corresponding second burl 240. A seal must be provided toensure that it is possible to generate different vacuum pressures ofeither side of the main body 210.

Passages are provided in order to generate the underpressures. A passagein the substrate support WT with an opening in the support surface 300is provided to generate the underpressure between the substrate holder200 and the substrate support WT. In order to generate the underpressurebetween the substrate holder 200 and the substrate W, one or morepassages within the main body 210 with openings in the first main bodysurface 212 are provided. The opposite ends of those passages areconnected to an underpressure source with a hose, for example.

Prior art substrate holders have required the distal end surfaces of allof the first burls to lie substantially in the same plane. Previously ithas been only been possible to achieve a flatness within specificationif each of the first burls 220 has a corresponding second burl 240. Thisis because the distal end surfaces of the first burls 220 where polishedin order to achieve the desired level of flatness. The amount ofmaterial removed during polishing depends upon the force between thepolishing stone and the first burl 220. In a case of a first burl 220not having a corresponding second burl 240, the polishing stone woulddepress the first burl 220 and slightly bend the main body 210 and soremove less material than from those first burls 220 than in the casewhere they are supported directly underneath by a corresponding secondburl 240. However since the introduction of new flattening techniques,such as ion beam milling/polishing, litho etching etc. the variation instiffness of the main body 210 under the first burl 220 no longerintroduces such a defect because the amount of material removed in ionbeam milling is not related to the stiffness of the main body 210 underthe burls being milled. The introduction of these new flatteningtechniques has enabled substrate holders 200 such as illustrated in FIG.4 to be introduced where burls of different subsets of first burls canbe manufactured with different heights.

In an embodiment the gap 222 is 3 nm or more, preferably 10 nm or moreand more preferably 30 nm or more, desirably 50 nm or more or even 100nm or more. The gap 222 is chosen according to the stiffness of the mainbody 210 and the magnitude of the first and second forces, in use,during imagining. For example, the gap is equal to the difference instiffness of the main body 210 between the burls of the first subset 220a and the second subset 220 b multiplied by the overall force applied tothe main body 210 by the first force and second force during imaginingof the substrate.

The above description is explained on the basis of the forces betweenthe substrate W, substrate holder 200 and substrate support WT beinggenerated using underpressures. However this need not be the case andthe forces may be generated in any other way, including by usingelectrostatic forces. This will now be described with reference to FIG.8.

In FIG. 8 electrodes 500, 510, 600, 610 are provided for generating thefirst and second forces between the substrate W, the substrate holder200 and the substrate support WT. The magnitude of the forces aredependent upon the voltage applied to the electrodes.

In the embodiment of FIG. 8 one or more substrate electrode(s) 500is/are deposited on the underside of the substrate W. One or more firstsubstrate holder electrode(s) 510 embedded within the main body 210 ofthe substrate holder 200 is/are also provided. By providing a potentialdifference between the substrate electrode(s) 500 and the firstsubstrate holder electrode(s) 510, a force attracting the substrate Wtowards the substrate holder 200 can be generated. Similarly, one ormore substrate support electrode(s) 600 is/are provided in the substratesupport WT. One or more second substrate holder electrode(s) 610 is/areprovided in the main body 210 of the substrate holder 200. A potentialdifference between the substrate support electrode(s) 600 and the secondsubstrate holder electrode(s) 610 results in a force between thesubstrate holder 200 and the substrate support WT. Apart from the way inwhich the force is generated the clamping system 1000 of FIG. 8 is thesame as described above with reference to FIGS. 3-7.

In an embodiment only one or more first substrate holder electrode(s)is/are provided in the substrate holder 200 which is/are at the samepotential and is/are used both for generating the force between thesubstrate W and the substrate holder 200 and between the substrateholder 200 and the substrate support WT.

FIGS. 9 and 10 show a further embodiment. In the embodiment of FIGS. 9and 10 the second force between the substrate holder 200 and substrate Wmay be generated either electrostatically as illustrated in FIGS. 9 and10 or using an underpressure as illustrated for example in FIG. 3 or bya different means or by a combination of those two means.

In the embodiment of FIGS. 9 and 10 the stiffness of the main body 210at positions corresponding to burls of the first subset 220 a and atpositions corresponding to burls of the second subset 220 b is the samealthough this is not necessarily the case. However at least two sets ofsubstrate holder electrodes 620, 630 are provided such that the firstforce at portions of the main body 210 of the substrate holder 200underneath the first subset 220 a are controllable independently of thefirst force at portions of the main body 210 of the substrate holder 200underneath the second subset 220 b of the plurality of first burls 220.In the embodiment of FIGS. 9 and 10 this is arranged by ensuring thatthe relative positions of the first burls 220 and the second burls 240alternate such that each first burl 220 is positioned, in plan, in a gapbetween corresponding second burls 240. Thus each of the first burls 220is at a position at which quite a large deformation of the main body 210is possible (i.e. the main body 210 is not stiff). By providing firstsubstrate holder electrodes 630 at positions underneath burls of thefirst subset 220 a and the second substrate holder electrodes 620 atpositions underneath burls of the second subset 220 b, it is possibleindependently to control the height of the distal end surfaces of burlsof the first subset 220 a relative to the support surface 300 comparedto the height of the distal end surfaces of burls of the second subset220 b relative to the support surface 300.

For example, as illustrated in FIG. 9, an electrostatic force isgenerated between second substrate holder electrodes 620 correspondingto burls of the second subset 220 b and an electrode 310 of thesubstrate support WT. As a result of this attractive force, the burls ofthe second subset 220 b are retracted away from the substrate W leavingthe substrate W supported by the burls of the first subset 220 a. Inthis position any in-plane stresses in the burls of the second subset220 b can relax.

Conversely, in FIG. 10, electrostatic forces generated between the firstsubstrate holder electrodes 630 and the electrode 310 which is part ofthe substrate support WT are effective to retract the burls of the firstsubset 220 a away from the substrate W. The substrate W is thensupported by burls of the second subset 220 b. Any in-plane stresses inthe burls of the first subset 220 a can relax. This process may berepeated any number of times. In order then to clamp the substrate W forimagining the distal end surfaces of all of the first burls 220 areallowed to contact the undersurface of the substrate W. In thisembodiment the distal end surfaces of all of the first burls 220 aresubstantially coplanar, even when no forces are present on the substrateholder 200.

FIG. 11 illustrates an embodiment of an arrangement of first burls 220and second burls 240 according to the invention. In the schematicdiagram the solid dots are representative of first burls 220 (of thefirst subset 220 a) which have no corresponding second burl 240 (e.g.like the central burl of FIGS. 5-8) whereas circles with no shading arerepresentative of first burls 220 (of the second subset 220 b) whichhave corresponding second burls 240 under them (i.e. like on the rightand left hand side of the embodiments shown in FIGS. 5-7).

In the FIG. 11 embodiment only one in every four of the first burls 220has a corresponding second burl 240. The supported area per first burl220 (i.e. the supporting area of second burls 240) is similar. Therotational symmetry of the first and second pattern (four) is the samefor the first pattern and second pattern. This desirably results insubstantially symmetrical deformation of the main body 210 around eachof the second burls 240.

Embodiments are provided according to the following clauses:

1. A method of clamping a substrate to a clamping system, the methodcomprising:

providing a substrate holder comprising;

-   -   a main body having a first main body surface and a second main        body surface, wherein the first main body surface and second        main body surface are on opposite sides of the main body; and    -   a plurality of first burls projecting from the first main body        surface, wherein each first burl has a distal end surface        configured to support the substrate;

providing a support surface for supporting the substrate holder;

providing a plurality of second burls for supporting the substrateholder on the support surface through contact with distal end surfacesof the plurality of second burls:

generating a first force to attract the substrate holder to the supportsurface;

placing the substrate on the substrate holder such that it contacts theplurality of first burls;

generating a second force to attract the substrate to the substrateholder; and

controlling at least one of the first force and the second force in arelease step to deform the main body between the second burls such as tocreate a gap between the distal end surfaces of a first subset of theplurality of first burls and the substrate and such that the substrateis supported on distal end surfaces of a second subset of the pluralityof first burls.

2. The method of claim 1, wherein the controlling further comprisescontrolling at least one of the first force and the second force in are-engagement step to deform the main body such that the distal endsurfaces of the first subset of the plurality of first burls contact thesubstrate.3. The method of claim 2, wherein the controlling further comprisescontrolling at least one of the first force and the second force in adisengagement step to deform the main body such as to create a gapbetween the distal surfaces of the second subset of the plurality offirst burls and the substrate.4. The method of claim 3, wherein the controlling further comprisescontrolling at least one of the first force and the second force in afurther re-engagement step to deform the main body such that the distalsurfaces of the second subset of the plurality of first burls contactthe substrate.5. The method of claim 4, wherein the release step, re-engagement step,disengagement step and further re-engagement step are performed in thatorder multiple times.6. The method of any of claims 1 to 5, wherein the release stepcomprises increasing a magnitude of the first force from an initialvalue and/or reducing a magnitude of the second force from an initialvalue.7. The method of any of claims 1 to 6, wherein the first force is atleast partly generated by applying an underpressure between the supportsurface and the substrate holder, and/or wherein the second force is atleast partly generated by applying an underpressure between thesubstrate and the substrate holder, and/or wherein the first forceand/or the second force is at least partly generated by applying anelectrostatic force via an electrode.8. The method of claim 7, wherein the main body is configured, in use,to block passage of gas from the first main body surface to the secondmain body surface through the main body and vice versa.9. A substrate holder for use in a lithographic apparatus and configuredto support a substrate on a substrate support, the substrate holdercomprising:

a main body having a first main body surface and a second main bodysurface, wherein the first main body surface and the second main bodysurface are on opposite sides of the main body;

a plurality of first burls projecting from the first main body surface,wherein each first burl has a distal end surface configured to supportthe substrate; and

a plurality of second burls projecting from the second main bodysurface, wherein each second burl has a distal end surface forsupporting the substrate holder on the substrate support,

wherein the distal end surfaces of a first subset of the plurality offirst burls are a first distance from the first main body surface andthe distal end surfaces of a second subset of the plurality of firstburls are a second distance from the first main body surface, the firstdistance is more than the second distance.

10. A substrate support comprising:

a substrate holder comprising:

-   -   a main body having a first main body surface and a second main        body surface, wherein the first main body surface and second        main body surface are on opposite sides of the main body; and    -   a plurality of first burls projecting from the first main body        surface, wherein each first burl has a distal end surface        configured to support the substrate; and

a support surface for supporting the substrate holder through contactwith distal end surfaces of a plurality of second burls projecting fromthe support surface;

wherein the distal end surfaces of a first subset of the plurality offirst burls are a first distance from the first main body surface andthe distal end surfaces of a second subset of the plurality of firstburls are a second distance from the first main body surface, the firstdistance is more than the second distance.

11. The substrate holder of claim 9 or the substrate support of claim10, wherein the main body is configured in use, to block passage of gasfrom the first main body surface to the second main body surface throughthe main body and vice versa.

12. The substrate holder of claim 9 or 11 or the substrate support ofclaim 10 or 11, wherein the second subset of the plurality of firstburls each has an axis which is closer to an axis of one of theplurality of second burls than an axis of the first subset of theplurality of first burls, and/or wherein there is a greater number ofthe first burls than the second burls, and/or wherein the number of thesecond subset of the plurality of first burls is substantially equal tothe number of the plurality of second burls.13. The substrate holder of any of claim 9, 11 or 12 or the substratesupport of any of claim 12, 13 or 14, wherein a stiffness of thesubstrate holder at a position corresponding to positions of each of thesecond subset of the plurality of first burls is greater than astiffness of the substrate holder at a position corresponding topositions of each of the first subset of the plurality of first burls.14. A substrate holder for use in a lithographic apparatus andconfigured to support a substrate on a substrate support, the substrateholder comprising:

a main body having a first main body surface and a second main bodysurface, wherein the first main body surface and the second main bodysurface are on opposite sides of the main body;

a plurality of first burls projecting from the first main body surface,wherein each first burl has a distal end surface configured to supportthe substrate;

a plurality of electrodes to which voltages may be applied in order tosecure the substrate holder to the substrate support; wherein theplurality of electrodes are configured such that depending on thevoltage applied, a force between portions of the main body of thesubstrate holder underneath a first subset of the plurality of firstburls and the substrate support is controllable independently of a forcebetween portions of the main body of the substrate holder underneath asecond subset of the plurality of first burls and the substrate support.

15. A lithographic apparatus comprising a substrate holder of any ofclaims 9 and 11 to 14 or the substrate support of any of claims 10 to13.

Although specific reference may be made in this text to the use of alithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Where the context allows, embodiments of the invention may beimplemented in hardware, firmware, software, or any combination thereof.Embodiments of the invention may also be implemented as instructionsstored on a machine-readable medium, which may be read and executed byone or more processors. A machine-readable medium may include anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computing device). For example, a machine-readablemedium may include read only memory (ROM); random access memory (RAM);magnetic storage media; optical storage media; flash memory devices;electrical, optical, acoustical or other forms of propagated signals(e.g. carrier waves, infrared signals, digital signals, etc.), andothers. Further, firmware, software, routines, instructions may bedescribed herein as performing certain actions. However, it should beappreciated that such descriptions are merely for convenience and thatsuch actions in fact result from computing devices, processors,controllers, or other devices executing the firmware, software,routines, instructions, etc. and in doing that may cause actuators orother devices to interact with the physical world.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A substrate holder for use in alithographic apparatus and configured to support a substrate on asubstrate support, the substrate holder comprising: a main body having afirst main body surface and a second main body surface, wherein thefirst main body surface and the second main body surface are on oppositesides of the main body; a plurality of first burls projecting from thefirst main body surface, wherein each first burl has a distal endsurface configured to support the substrate; and a plurality of secondburls projecting from the second main body surface, wherein each secondburl has a distal end surface for supporting the substrate holder on thesubstrate support, wherein the distal end surfaces of a first subset ofthe plurality of first burls are a first distance from the first mainbody surface and the distal end surfaces of a second subset of theplurality of first burls are a second distance from the first main bodysurface, the first distance is more than the second distance.
 2. Thesubstrate holder of claim 1, wherein the main body is configured in use,to block passage of gas from the first main body surface to the secondmain body surface through the main body and vice versa.
 3. The substrateholder of claim 1, wherein the second subset of the plurality of firstburls each has an axis which is closer to an axis of one of theplurality of second burls than an axis of the first subset of theplurality of first burls, and/or wherein there is a greater number ofthe first burls than the second burls, and/or wherein the number of thesecond subset of the plurality of first burls is substantially equal tothe number of the plurality of second burls.
 4. The substrate holder ofclaim 1, wherein a stiffness of the substrate holder at a positioncorresponding to positions of each of the second subset of the pluralityof first burls is greater than a stiffness of the substrate holder at aposition corresponding to positions of each of the first subset of theplurality of first burls.
 5. The substrate holder of claim 1, whereinthe first distance is 3 nm or more than the second distance.
 6. Asubstrate support comprising: a substrate holder comprising: a main bodyhaving a first main body surface and a second main body surface, whereinthe first main body surface and second main body surface are on oppositesides of the main body; and a plurality of first burls projecting fromthe first main body surface, wherein each first burl has a distal endsurface configured to support the substrate; and a support surfaceconfigured to support the substrate holder through contact with distalend surfaces of a plurality of second burls projecting from the supportsurface, wherein the distal end surfaces of a first subset of theplurality of first burls are a first distance from the first main bodysurface and the distal end surfaces of a second subset of the pluralityof first burls are a second distance from the first main body surface,the first distance is more than the second distance.
 7. The substratesupport of claim 6, wherein the main body is configured in use, to blockpassage of gas from the first main body surface to the second main bodysurface through the main body and vice versa.
 8. The substrate supportof claim 6, wherein the second subset of the plurality of first burlseach has an axis which is closer to an axis of one of the plurality ofsecond burls than an axis of the first subset of the plurality of firstburls, and/or wherein there is a greater number of the first burls thanthe second burls, and/or wherein the number of the second subset of theplurality of first burls is substantially equal to the number of theplurality of second burls.
 9. The substrate support of claim 6, whereina stiffness of the substrate holder at a position corresponding topositions of each of the second subset of the plurality of first burlsis greater than a stiffness of the substrate holder at a positioncorresponding to positions of each of the first subset of the pluralityof first burls.
 10. The substrate support of claim 6, wherein the firstdistance is 3 nm or more than the second distance.
 11. A substrateholder for use in a lithographic apparatus and configured to support asubstrate on a substrate support, the substrate holder comprising: amain body having a first main body surface and a second main bodysurface, wherein the first main body surface and the second main bodysurface are on opposite sides of the main body; a plurality of firstburls projecting from the first main body surface, wherein each firstburl has a distal end surface configured to support the substrate;protruding portions projecting from the second main body surface; and aplurality of electrodes to which voltages may be applied in order tosecure the substrate holder to the substrate support, at least one ofthe electrodes extending at least partly in a region between theprotruding portions, wherein the plurality of electrodes are configuredsuch that depending on the voltage applied, a force between thesubstrate support and portions of the main body of the substrate holderunderneath a first subset of the plurality of first burls iscontrollable independently of a force between the substrate support andportions of the main body of the substrate holder underneath a secondsubset of the plurality of first burls.
 12. The substrate holder ofclaim 11, wherein the main body is configured in use, to block passageof gas from the first main body surface to the second main body surfacethrough the main body and vice versa.
 13. The substrate holder of claim11, wherein a stiffness of the substrate holder at a positioncorresponding to positions of each of the second subset of the pluralityof first burls is greater than a stiffness of the substrate holder at aposition corresponding to positions of each of the first subset of theplurality of first burls.
 14. A method of clamping a substrate to aclamping system, the method comprising: generating a first force toattract a substrate holder to a support surface for supporting thesubstrate holder, wherein the substrate holder comprises a main bodyhaving a first main body surface and a second main body surface, whereinthe first main body surface and second main body surface are on oppositesides of the main body, and a plurality of first burls project from thefirst main body surface, wherein each first burl has a distal endsurface to support a substrate and wherein a plurality of second burlssupport the substrate holder on the support surface through contact withdistal end surfaces of the plurality of second burls; placing thesubstrate on the substrate holder such that it contacts the plurality offirst burls; generating a second force to attract the substrate to thesubstrate holder; and controlling the first force and/or the secondforce in a release step to deform the main body between the second burlssuch as to create a gap between the distal end surfaces of a firstsubset of the plurality of first burls and the substrate and such thatthe substrate is supported on distal end surfaces of a second subset ofthe plurality of first burls.
 15. The method of claim 14, wherein thecontrolling further comprises controlling the first force and/or thesecond force in a re-engagement step to deform the main body such thatthe distal end surfaces of the first subset of the plurality of firstburls contact the substrate.
 16. The method of claim 15, wherein thecontrolling further comprises controlling the first force and/or thesecond force in a disengagement step to deform the main body such as tocreate a gap between the substrate and the distal surfaces of the secondsubset of the plurality of first burls.
 17. The method of claim 16,wherein the controlling further comprises controlling the first forceand/or the second force in a further re-engagement step to deform themain body such that the distal surfaces of the second subset of theplurality of first burls contact the substrate.
 18. The method of claim17, wherein the release step, re-engagement step, disengagement step andfurther re-engagement step are performed in that order multiple times.19. The method of claim 14, wherein the release step comprisesincreasing a magnitude of the first force from an initial value and/orreducing a magnitude of the second force from an initial value.
 20. Themethod of claim 14, wherein the first force is at least partly generatedby applying an underpressure between the support surface and thesubstrate holder, and/or wherein the second force is at least partlygenerated by applying an underpressure between the substrate and thesubstrate holder, and/or wherein the first force and/or the second forceis at least partly generated by applying an electrostatic force via anelectrode.